Medium expansion foams are generally used to quench burning root system following a forest fire, to extinguish grass fires, to apply fire barrier blanket strips around buildings and wood structures such as bridges and towers, and to extinguish car fires by rapidly filling the vehicle with high volume foam.
Fire fighting foam is produced by expanding a fluid, usually water mixed with a foaming agent, through a nozzle, and by introducing air into the expanding spray.
The science of thermodynamics teaches that a nozzle consists of a throat region having an opening area smaller than the area of the supply conduit. A nozzle also consists of a convergent section at the entry of the throat, and may have a divergent section following the throat.
Such nozzle is used primarily to reduce the pressure of a fluid, to increase its velocity, and, of pertinent importance, to increase the specific volume of the fluid.
The efficiency in expanding a fluid through a nozzle is directly related to a suitable combination of an area ratio: (exit area/throat area) and a pressure ratio: (discharge pressure/throat pressure).
Along the same line of teaching, an elongated divergent section following a nozzle is generally known as a diffuser. A diffuser tends to reduce the velocity of the fluid and increases its discharge pressure. The combination of convergent nozzle and diffuser is known in the art as a venturi.
Furthermore, in fluid dynamics, an injector generally functions as a device which uses the kinetic energy of one fluid to pump another fluid from a region of lower pressure.
The region of lower pressure for placement of an air injector in a foam nozzle being as close as possible from the discharge side of the throat, the pressure gradient through the diffuser is directly proportional to the efficiency of the injector. The physical dimensions, the angle of divergence of a diffuser and the rate of expansion of the fluid are therefore other factors which require optimization in order to design an efficient foam producing venturi.
U.S. Pat. No. 4,830,790, issued to Douglas E. Stevenson discloses two types of nozzles. The first one is a low expansion foam nozzle having apertured plate to promote turbulence in the fluid at the entry of the throat. The air injector holes are located partly on a convergent section of the throat. The nozzle has a tubular diffuser intended to increase throw distance of the foam rather than maximizing foam expansion.
The second nozzle disclosed by Stevenson is a medium expansion foam nozzle. The nozzle has also a tubular diffuser. The injector holes are placed near the larger end of the divergent section, and therefore at some distance from the minimum pressure region. In this embodiment, the apertured plates serve both purposes of a turbulence enhancer and a throat orifice.
U.S. Pat. No. 5,054,688, issued to John R. Grindley discloses another low expansion nozzle having venturi type orifices, radial injector openings and a tubular diffuser.
The disclosed foam nozzles as well as other models available commercially, having tubular diffusers may be somewhat efficient where the reach of the material from the nozzle is more important than ideal foam expansion.
Moreover, previous foam nozzles for medium expansion were found to operate satisfactorily only within a very narrow range of supply pressure, typically from 75 to 95 psi.
This narrow operational pressure range of commercial medium expansion nozzles represents a substantial inconvenience for fire fighting applications. A fire truck can generally deliver pressures of over 300 psi, and firemen arriving at a burning site usually do not have the time to regulate the hose pressure to accommodate the nozzle requirement. Adding the complication of pressure losses from several lengths of hoses, or from the elevation of the nozzle, the ideal pressure conditions may sometimes become difficult to obtain.
Furthermore, it is not a common practice to "choke" valves during a fire fighting operation. Consequently, the pressure setting for commercial medium expansion foam nozzles is widely ignored, and the aeration of the foaming agent is not always optimum. It is therefore common to spray a foam which has the texture of pearly white water, which dissipates rapidly, and which drips without having performed as anticipated while using an inordinate amount of foaming agent.
The low performance of the existing nozzles outside the pressure range they are capable of handling may be due to area/pressure ratio, location and size of injector openings, and the dimensions of the diffuser.
Thermodynamics and fluid mechanics indicate that the friction of the fluid against the wall of the diffuser, and the rapid expansion of an aerated mixture can create a compression zone within the diffuser. A compression zone has the adverse effect of saturating the expansion process, causing the foam to condense before reaching the discharge end of the diffuser. Thus, the shape and dimensions of the diffuser becomes very important to avoid formation of such a compression region.
It will be evident that the venturi, according to the present invention, may be used in a host of applications where a voluminous and rich foam is desired or where a fluid required significant aeration. Examples of additional applications include foamable insulation distribution, distribution of detergents, absorbents, herbicides, insecticides, etc.